Using IPCC simulations of air flow, Cassou has shown that global warming will increase summer blocking events over Europe.

Blocking occurs when the jet stream, which carries winds from the west, is forced to slow down suddenly. “It catches up on itself and starts to meander,” says Mike Lockwood from the University of Reading, UK. Sometimes the meanders double back on themselves, allowing north-easterly winds to fill the gap.

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When solar activity is low this jet stream “pile-up” shifts eastwards across the Atlantic Ocean, bringing blocking events to Europe. The reasons seems to be that solar activity influences high-level stratospheric winds, and these eventually feed through to the troposphere, where the jet stream lies.

Sun winding down

“Solar activity tends to ramp up for 300 to 400 years and then fall again over about 100 years,” says Lockwood. Right now the sun has just begun its downward path from a maximum, suggesting that blocking patterns will become more common over Europe during the next century.

Global warming may compound the problem. “As the troposphere becomes warmer you get more vertical mixing but less horizontal mixing, making it easier for a blocking event to occur,” says Julian Hunt, a climate scientist at University College London. The lack of horizontal mixing makes it easier for weather systems to sit in one place.

Hunt’s research suggests that the problem will become particularly acute in summers to come, and that blocking events may become more frequent and sit over Europe for 20 days or more.